One of the many measurements made of an internal combustion engine to evaluate its performance is the peak compression in the various compression chambers. If the readings do not meet manufacturers' specifications, the mechanic knows that the engine is not functioning properly. He then follows a standardized procedure to correct the problem so that the peak compression in each chamber does meet the specifications.
Currently, peak compression readings are obtained for a given cylinder by removing the associated spark plug and replacing it with a compression gauge. Such instrument has a threaded end matching the threaded end of the spark plug so that it can be screwed into the spark plug opening of the cylinder. The gauge has a pointer rotatable on a dial calibrated in pounds per square inch. The automobile starter is operated to cause the pistons to operate, thereby varying the compression in the cylinder. The mechanic watches the gauge as the pointer moves between the minimum and maximum positions caused by the increases and decreases of the compression in the cylinder. He notes the maximum reading, which corresponds to peak compression in that cylinder. He performs similar steps for each cylinder in the engine. In this manner, a mechanic is able to make reasonably accurate readings of peak compression in the cylinders of a standard internal combustion engine.
However, in a rotary internal combustion engine, each spark plug is associated with three combustion chambers rather than the single combustion chamber in the cylinder of a standard engine. As the rotor of a rotary engine spins, it presents to the associated spark plug, three different combustion chambers, one after another. A mechanical gauge as described above inserted in place of the spark plug would be difficult, it not impossible, to read accurately, particularly since peak compression for each chamber may not be the same. The mechanic would have to watch the gauge as the pointer moves back and forth through peaks at the rate of 15 to 20 times a second.
There have been efforts directed to designing a compression measuring apparatus that is electronic, that is to say, apparatus that produces electrical signals corresponding to the compressions in an internal combustion engine. Such efforts have not, however, been usable in rotary engines. For example, U.S. Pat. No. Re. 29,109 to Petersen et al., for Apparatus for the Determination of Internal Combustion Engine Compression Pressure, discloses a device of this nature for measuring peak compression in a standard internal combustion engine. This patent does not disclose how to obtain accurate digital readings of the compression. Also, the Petersen et al. apparatus suffers the disadvantages of not having means to apprise the mechanic that the information on compression has been stored. The use of engine current to provide electrical indication of compression, as suggested by this patent, undesirably causes the readings to be affected by extraneous factors such as battery voltage and ground series resistance. Finally, the use of delay as a means to insure that the engine has reached steady state, as disclosed in the patent, is not sufficiently accurate.